Honestly what I suggest is removing that small sound from the map and exporting the map. If it works, then it's definetly something related with that small sound. I then suggest adding that sound back, and then removing a different sound from the map and try exporting again. Reason I want you to test this is to see whether or not the small sound is the issue, or whether it's something to do with the overall size of the export as you think could be the issue.
Come back once you've done both and let me know the results of each test.
The size must have been the problem after all then! Happy to hear it's working fine for you and your friend now, though!